Norrander J M, Perrone C A, Amos L A, Linck R W
Department of Cell Biology and Neuroanatomy, University of Minnesota, Minneapolis, 55455, USA.
J Mol Biol. 1996 Mar 29;257(2):385-97. doi: 10.1006/jmbi.1996.0170.
Recent structural studies indicate that a tektin heteropolymer forms a unique protofilament of flagellar microtubules. We report here the sequence of tektin C (approximately 47 kDa), predicted from its cDNA (GenBank U38523), compared to tektins A (approximately 53 kDa) and B (approximately 51 kDa) from sea urchin (Strongylocentrotus purpuratus) sperm flagellar microtubules, and compared to partial sequences reported from mouse and human. We are now able to make several observations concerning the tektin family: (1) their common structural features, (2) a comparison of their structure to intermediate filament proteins, and (3) their possible organization in the tektin filament polymer. The predicted amino acid sequence identities/similarities are: for tektins A and C, 42/54%, for tektins A and B, 34/51%; for tektins B and C, 29/42%; for tektin C and a partial cDNA clone from mouse testis, 55/65%; and for tektin B and a partial cDNA clone from the human brain, 45/47%. The three tektins (and the human clone) possess the exact sequence repeat RPNVELCRD. The structural pattern of all three tektin polypeptides is similar to intermediate filament proteins. Tektins are predicted to form extended rods composed of two alpha-helical segments (approximately 180 residues long) capable of forming coiled coils, which are interrupted by short non-helical linkers. The two segments are homologous in sequence and secondary structure, indicating a gene duplication event prior to the divergence of the three tektins. Along each tektin rod cysteine residues occur with a periodicity of approximately 8 nm, coincident with the axial repeat of tubulin dimers in microtubules. From EM data and calculations of secondary structure, the segment length of tektin AB heterodimers is likely to be 16 nm. Both segments of tektin C may be 24 nm long, but one may be 16 nm. On the basis of the available evidence, we propose that coassembly of tektin AB heterodimers with tektin C dimers produces filaments with overall repeats of 8, 16, 24, 32, 40, 48 and 96 nm, generating the basis for the complex spatial arrangements of axonemal components.
最近的结构研究表明，一种微管蛋白异源聚合物形成了鞭毛微管独特的原纤维。我们在此报告从其cDNA（基因库U38523）预测的微管蛋白C（约47 kDa）的序列，将其与海胆（紫球海胆）精子鞭毛微管中的微管蛋白A（约53 kDa）和B（约51 kDa）进行比较，并与从小鼠和人类报道的部分序列进行比较。现在我们能够对微管蛋白家族做出几点观察：（1）它们的共同结构特征，（2）将它们的结构与中间丝蛋白进行比较，以及（3）它们在微管蛋白丝聚合物中的可能组织方式。预测的氨基酸序列同一性/相似性为：微管蛋白A和C为42/54%，微管蛋白A和B为34/51%；微管蛋白B和C为29/42%；微管蛋白C与小鼠睾丸的一个部分cDNA克隆为55/65%；微管蛋白B与人类大脑的一个部分cDNA克隆为45/47%。这三种微管蛋白（以及人类克隆）具有完全相同的序列重复RPNVELCRD。所有三种微管蛋白多肽的结构模式与中间丝蛋白相似。预测微管蛋白形成由两个α螺旋段（约180个残基长）组成的延伸杆，能够形成卷曲螺旋，中间被短的非螺旋连接区打断。这两个段在序列和二级结构上是同源的，表明在三种微管蛋白分化之前发生了基因复制事件。沿着每个微管蛋白杆，半胱氨酸残基以约8 nm的周期出现，与微管中微管蛋白二聚体的轴向重复一致。根据电子显微镜数据和二级结构计算，微管蛋白AB异二聚体的段长可能为16 nm。微管蛋白C的两个段可能都为24 nm长，但也可能一个为16 nm。根据现有证据，我们提出微管蛋白AB异二聚体与微管蛋白C二聚体的共组装产生具有8、16、24、32、40、48和96 nm整体重复的丝，为轴丝成分的复杂空间排列奠定基础。
